Violence across Syria on Sunday left at least 40 people dead, including 35 in the northwest, Rami Abdel Rahman, head of the London-based Syrian Observatory for Human Rights, told AFP on Monday.
Thirty-five people -- 27 civilians and eight security agents -- were killed in Jisrash Shugur as military and security forces continued operations against anti-regime protesters in northwestern Idlib province.
Further north, in the town of Idlib, security forces scattered some 1,500 demonstrators, he said.
Two civilians were also reported killed in the Mediterranean coastal town of Jabla when security forces opened fire to disperse demonstrators calling for the release of a detained sheikh.
In the eastern town of Deir Ezzor, security forces shot at protesters marching in front of a building of the ruling Baath party, killing three of them, Rahman said.
On Sunday, security forces returned the body of a man kept in custody for a month to his parents in the Damascus suburb of Douma.
Six human rights groups within Syria on Monday issued a joint statement condemning "the excessive use of force to disperse peaceful gatherings of unarmed Syrian citizens."
The groups call on the government to "stop the spiral of violence and assassinations in the streets of Syria."
They also demanded an independent and transparent commission of inquiry "to unmask those responsible for the violence."
Rights groups say more than 1,100 civilians have been killed and at least 10,000 arrested in Syria since protests against the government of President Bashar al-Assad erupted in mid-March.
Damascus insists the unrest is the work of "armed terrorist gangs" backed by Islamists and foreign agitators.